Inadequate pre-pregnancy BMI and gestational weight gain (GWG) have been associated with sub-optimal child development. We used data from the 2015 Pelotas (Brazil) Birth Cohort Study. Maternal anthropometry was extracted from antenatal/hospital records. BMI (kg/m2) and GWG (kg) adequacy were classified according to WHO and IOM, respectively. Development was evaluated using the INTER-NDA assessment tool for 3,776 children aged 24 months. Suspected developmental delay (SDD) was defined as <10th percentile. Associations between maternal exposures and child development were tested using linear and logistic regressions. Mediation for the association between BMI and child development through GWG was tested using G-formula. Sex differences were observed for all child development domains, except motor. Maternal pre-pregnancy underweight increased the odds of SDD in language (OR: 2.75; 95%CI: 1.30-5.80), motor (OR: 2.28; 95%CI: 1.20-4.33), and global (OR: 2.14; 95% CI: 1.05-4.33) domains for girls; among boys, excessive GWG was associated with SDD in language (OR: 1.59; 95%CI: 1.13-2.24) and cognition (OR: 1.59; 95%CI: 1.15-2.22). Total GWG suppressed the association of pre-pregnancy BMI with percentiles of global development in the entire sample. Maternal underweight and excessive GWG were negatively associated with development of girls and boys, respectively. The association of pre-pregnancy BMI with global child development was not mediated by GWG, irrespective of child's sex.